1. Technical Field of the Invention
The present invention in some embodiments refers to a tank system for providing an ice-free cold start volume of an operating liquid required by a liquid consumer after a cold start. A SCR (selective catalytic reduction) catalytic converter comprises a main tank having a filling opening, through which the interior of the main tank can be filled with operating liquid, such as a urea solution. The invention in some embodiments further refers to a kit for such tank systems.
2. Description of the Related Art
In fluid systems, which convey a liquid from a reservoir through lines to a liquid consumer, the problem occurs that the liquids freeze in the tanks if the ambient temperature drops below the freezing point of the liquid to be conveyed. During the winter, for example, the cleaning solution of the windscreen washer system freezes in the tank so that the rear window and the windshield cannot be cleaned immediately after the cold start.
For this reason, the cleaning liquids of windscreen and headlamp washing systems in motor vehicles are supplied with an antifreeze. However, the antifreeze only lowers the freezing point of the windscreen wiping water to approx. −20° C. to −17° C. so that the liquid freezes in the tank and in the tubes of the windscreen wiping system at temperatures below −20° C. despite the antifreeze.
To lower the nitrogen oxide emissions in the exhaust gases of internal combustion engines, e.g. of diesel engines, an exhaust gas purification according to the so-called SCR method may be carried out. In the SCR method, the nitrogen oxides are chemically converted in a catalytic converter with a suitable reduction agent to the harmless substances, such as nitrogen and water. As a reduction agent, vaporous or gaseous ammonium is used, which is generated from an aqueous urea solution and which is introduced into the exhaust gas flow.
Under the trade name AdBlue® an aqueous urea solution with a urea content of 32.5 percent by weight for reducing the nitrogen oxide emission in motor vehicles is offered. Using AdBlue® the problem exists that this liquid freezes below −11° C. in the tank of the SCR system and can no longer be conveyed to the catalytic converter.
Thus, it is required to equip the vehicle tanks of SCR systems or windscreen or headlamp cleaning systems with heating systems, which melt the frozen liquid in the tank, i.e., transform it to a liquid state capable of flowing to the consumer.
Systems are known from the prior art, in which liquid tanks in motor vehicles are externally coated by heating sheets or heating mats to melt the liquid frozen in the tank. Other heating systems are for example shown in DE 203 15 852 U, DE 195 08 598 A, US 2002/088220 A1 and US 2005/177696 A1.
However, heating sheets or heating mats applied to the outside of the tank wall have a very poor efficiency, since a major part of the heat generated is not conducted into the tank to melt the frozen liquid but gets lost to the environment of the tank.
The development of tank heating systems aims at accommodating heating coils within the tank to avoid the loss of heat occurring with the heating mats. However, the use of heating coils within the tank is disadvantageous since the heat of the heating coils is only distributed to a relatively small volume. Since the melting by means of heating coils takes place mostly in the direct proximity of the heating surface, layers of air are formed around the radiator when the water runs off. These air cushions have the disadvantage that they have a thermally insulating effect, and thus the efficiency of the heating coils drops drastically.
Additionally, the capacity and the temperature of the heating elements may not be arbitrarily high. If the heating temperature exceeds the boiling point of either the substance to be melted or of the component of the liquid with the lowest boiling point (e.g., the alcohol of the windscreen cleaning liquid), evaporation occurs in the proximity of the heating surfaces. Air bubbles are generated during this evaporation, which also reduces the heat conduction and the efficiency of the melting procedure. During melting of aqueous urea solutions, temperatures of above 60° C. lead to a thermal decay of the urea so that this maximum temperature at the exothermic surfaces must not be exceeded.